Andrew H. Cobb

Factors influencing the free amino acid content of potato (Solanum tuberosum L) tubers during prolonged storage

Tubers of the potato cultivars Pentland Dell and Record were stored for a period of up to 40 weeks at 5°C and 10°C over four consecutive storage seasons (1989-1990 to 1992-1993). Reconditioning treatments at 20°C were also performed at early, mid and late stages in storage. To assess the turnover of proteins during storage, tubers were analysed for free amino acid and soluble protein content. The net direction of nitrogen flow was dependent on the state of dormancy of the tuber and hence storage duration. An increase in free amino acid content commonly occurred during the latter part of storage, caused by an upturn of proteinase activity on the break of dormancy. This increased enzyme activity was probably due to de novo synthesis of proteinases, in particular of a 47 kDa aspartic proteinase which was purified to homogeneity and shown to have a marked substrate-specificity for endogenous tuber proteins such as patatin. The rate of protein turnover increased with storage temperature, although the net direction of nitrogen flow was temperature independent. However, when nitrogen flow exhibited a distinct direction, such as the protein breakdown in Pentland Dell during late storage, this was enhanced by higher temperatures. The accumulation of free amino acids in Pentland Dell at 10°C corresponded with a deterioration of processing quality that could not be accounted for by an upturn in reducing sugar content. Despite an excess of free amino acids with respect to reducing sugars, amino acids had a probable synergistic influence on fry colour over the later stages of storage resulting in a darker colour per unit reducing sugar than in early storage. Reconditioning treatments were ineffective as a means of lowering the free amino acid pool size, these treatments operating solely through the decrease in reducing sugar content.

Is there a link between greening and light-enhanced glycoalkaloid accumulation in potato (Solanum tuberosumL) tubers?

Potato (Solanum tuberosum L) tubers (cvs Pentland Dell and Record) were treated with the chlorophyll biosynthesis inhibitors 4-amino-5-fluoropentanoic acid (AFPA) and 3-amino-2,3-dihydrobenzoic acid (gabaculine), and subsequently exposed to daylight for up to 10 days prior to pigment and glyco-alkaloid analysis. AFPA inhibited the accumulation of total chlorophyll (Chl) by between 50 and 70% in both cultivars throughout the duration of light exposure. The synthesis of Chl b was inhibited by over 80% in both cultivars. Neither inhibitor had a significant effect on light-enhanced glycoalkaloid accumulation. It is concluded that there is no direct metabolic link between Chl and glycoalkaloid biosynthesis.

Aspects of amino acid metabolism in stored potato tubers (cv. Pentland Dell)

Tubers of the cv. Pentland Dell were stored at 5 and 10°C for up to 33 weeks and analysed for soluble protein and free amino acids. In addition, glutamine synthetase, NADH-GOGAT and acid proteinase activities were measured over the 33-week storage period. An accumulation of asparagine and glutamine occurred during late storage which coincided with an upturn in proteinase activity. The amide content of the major tuber storage proteins suggested that amidation of free amino acids must account for the high proportion of free amides. The activities of glutamine synthetase and NADH-GOGAT further indicated amidation of free amino acids throughout storage, independent of storage temperature. An increase of total soluble protein during storage suggested that much of the observed increase in amides resulted from translocation into the tuber core tissue, perhaps as a result of the chemical suppression of sprout growth.

The effect of prior storage on the potential of potato tubers (Solanum tuberosum L) to accumulate glycoalkaloids and chlorophylls during light exposure, including artificial neural network modelling

Potato tubers of four varieties (Brodick, King Edward, Pentland Dell and Record) were stored under commercial conditions and exposed to light for up to 10 days after 0, 10, 20 and 30 weeks. These were analysed for photosynthetic pigment and glycoalkaloid content. There was no significant alteration in either tuber chlorophyll or glycoalkaloid content during dark storage. All four varieties greened in response to light exposure, but only three exhibited a significant increase in glycoalkaloid concentrations during this exposure. Storage duration did not significantly affect pigment accumulation. However, there was a marked effect of storage on the extent of glycoalkaloid accumulation. Tubers of all four varieties stored for more than 10 weeks did not accumulate glycoalkaloids to the same extent as fresh tubers. Indeed, Brodick and Record did not accumulate any glycoalkaloids in response to light after 30 weeks of storage. A number of artificial neural network models of the results were produced. These accurately modelled cultivars individually, but a model encompassing all the data was not successful at predicting cultivar differences.

Impact‐induced blackspots and membrane deterioration in potato (Solanum tuberosum L) tubers

Tubers of the potato cultivar Maris Piper (MP) were grown in 1996 and stored for 8 months, whereas tubers of the cultivar Colmo (CM) were grown in 1997 and harvested immature. In a timecourse study the two cultivars, which had two distinct physiological ages, were investigated for blackspot susceptibility in relation to cellular integrity. Tubers were equilibrated at 5 °C and struck by a falling bolt. The site of impact was analysed for cation efflux from tuber half-discs and enzymatic activity of polyphenoloxidase (PPO), lipoxygenase (LOX), ascorbate peroxidase (APX) and catalase after 0, 1, 2, 4, 8, 16 and 32 h of tuber incubation. Tubers of MP were 10 times more blackspot-susceptible than those of CM. The membranes of MP leaked more K+, Mg2+ and Ca2+ than those of CM. Both cultivars showed a very fast increase in K+ leakage immediately after impact. Thereafter, CM seemed to reconstitute the membranes, while high efflux of K+ from MP continued for up to 32 h of incubation. Mg2+ efflux from MP increased and peaked 8 h after impact, but no effect was found on CM. Ca2+ and Na+ effluxes were not influenced by the impact. More than twice the activity of LOX and more than three times the activity of PPO were found in CM compared to MP, but little effect of impact was registered. The activity of APX was higher in MP than in CM, and again little effect of impact was found. Little effect of impact on catalase activity was found and the level of catalase activity was similar in both cultivars. © 2000 Society of Chemical Industry

Phenmedipham-ozone pollution interactions in sugarbeet (Beta vulgaris L. cv. Saxon) : A physiological study

Actively growing sugarbeet is treated with the post-emergent herbicide phenmedipham at times when ozone pollution episodes are likely to occur. There is a possibility of an interaction occurring between ozone and phenmedipham as both treatments produce similar effects in susceptible plants, such as a reduction in growth and photosynthesis and an increase in the activities of endogenous antioxidant enzymes. To investigate this likelihood, laboratory experiments were conducted in which two- to three-leaf sugarbeet plants (Beta vulgaris L. cv. Saxon) were exposed to a simulated two-day ozone episode (100 nl litre -1 , 7 h day -1 ) followed three days later by treatment with field rate phenmedipham (1.14 kg AI ha -1 ). Growth analysis indicated that an interaction was occurring in which plants treated with ozone and phenmedipham had less reduction in shoot fresh weight than expected. Exposure to phenmedipham alone or ozone followed by phenmedipham reduced net photosynthesis by over 50% and transpiration rate by 30%. The activities of antioxidant enzymes such as catalase, guaiacol peroxidase and superoxide dismutase were stimulated by both treatments individually, but to a greater extent when ozone and phenmedipham were combined. For example, three days after herbicide treatment, the activity of superoxide dismutase increased by 20% in plants treated with ozone alone, 20% in plants treated with phenmedipham alone and 85% in plants that were treated with ozone followed by phenmedipham. We conclude that ozone pollution may predispose sugarbeet to tolerate the herbicide phenmedipham by enhancing the activity of the endogenous antioxidant detoxification enzyme system.

Ozone pollution modifies the response of sugarbeet to the herbicide phenmedipham

Experiments were conducted in which sugarbeet plants (Beta vulgaris L. cv. Saxon) with 2 to 3 leaves were exposed to a simulated 2 day ozone episode (100 nl l−1, 7 h d−1). Three days later, the plants were sprayed with field rate phenmedipham (1.14 kg a.i. ha−1) and growth analysis conducted 7 days later indicated an antagonistic interaction was occurring. Physiological and biochemical studies were undertaken to determine the nature of this antagonism. Treatment with phenmedipham increased the ratio of transpiration to photosynthetic rates within 2 days of spraying, whilst exposure to ozone had no effect. When the two treatments were combined, water use efficiency was not significantly different from that when phenmedipham was applied alone. In contrast, trends in the membrane permeability after treatment, indicated that the response of plants exposed to ozone followed by treatment with the herbicide, was intermediate between that of the herbicide (high permeability) and ozone (low permeability). Furthermore, when the two treatments were combined the results of antioxidant enzyme assays indicated greater than expected activities of enzymes which are mainly cytosolic, eg. guaiacol peroxidase, as well as a similar increase in the activity of the mainly chloroplastic superoxide dismutase. Treatment with ozone alone and phenmedipham alone only slightly increased superoxide dismutase. Ozone may therefore induce the activities of these protective enzymes. Thus, when another oxidative stress, such as the photosystem II inhibitor phenmedipham, was applied the plants could then respond more quickly and showed less herbicide visible damage.

Physiological changes in Matricaria inodora following ioxynil and bromoxynil treatment

A range of biochemical and physiological changes were monitored in Matricaria inodora following field rate applications of ioxynil and bromoxynil. Bromoxynil showed greater phytotoxicity with decreased reducing sugars, amino acids and proteins occurring within 28 days of treatment, when plant death was apparent. After an initial decline these parameters increased as plants appear to recover from ioxynil treatment. CO2 fixation was completely inhibited within 4 days in leaves treated with ioxynil and bromoxynil. Ultrastructural effects of both herbicides were similar with chloroplast swelling, decrease in starch grains and thylakoid disruption prior to cellular destruction. In vitro activity of the herbicides on isolated chloroplasts revealed ioxynil to be slightly more inhibitory than bromoxynil toward electron transport and approximately one hundred times more effective as an uncoupler of PMS cyclic photophosphorylation.